This invention relates to audio frequency power line carrier control systems in general and more particularly to such a system utilizing an inverter in which modulation of the inverter output is accomplished during start-up to allow using smaller series inductances.
Power line carrier control systems permit coded information to be transmitted through a three-phase or a single phase network. Low frequency waves in the frequency range between 150 and 500 Hz. are used for this purpose. These pulse trains are transmitted for a duration of time determined by a clock pulse. Receivers coupled to the distribution network are tuned to the respective audio frequency and decode the transmitted information. Information can be used, for example, to initiate switching operations in distribution networks, for switching a consumers meter to a different rate, e.g., night rate, or to notify a particular group of persons such as the fire department.
Audio frequency power line control systems utilizing inverters as the power line control transmitter with the output phase voltages fed into a distribution network through a series inductance, a filter circuit and a current transformer are known. However, in these devices the design of the series inductance is a problem. Ideally, this inductance should be as small as possible so that it will bring about good attenuation of the harmonics in the output voltage of the inverter and cause no excessive voltage under load. On the other hand, the series inductance must be made as large as possible in the known audio frequency power line control systems using inverters since the rate of rise of the inrush current upon starting up at the switching on of each clock pulse must be limited. This inrush current occurring during switching is caused by the charging of the capacitors in the filter circuit with a possible contribution due to the fact that a low impedence network is being fed. This inrush current leads to a transient with very high current peaks which the inverter must be designed to withstand.
In view of these difficulties, the need for a new means of operating an audio frequency power line control system of this nature which limits the inrush current without using an extremely large series inductance becomes evident.